Anger was also directed at May’s close circle of aides, Nick Timothy and Fiona Hill, who ran a campaign excluding senior cabinet ministers and oversaw the writing of the manifesto.

At her count in Maidenhead, May suggested the Conservatives would try to hang on as a minority government if they won the most seats but any failure to match David Cameron’s 330 seats in 2015 could be fatal for her leadership.

The former chancellor George Osborne, who was sacked by May last year, was one of the first senior party figures to react to the result, saying if the exit poll were correct it would be “catastrophic” for the Tories and the prime minister personally.

He described the Conservative manifesto as “one of the worst manifestos in history” and expressed disappointment that the party was perceived as turning away from metropolitan liberal voters.

“It’s difficult to see, if these numbers are right, how they would put together the coalition to remain in office,” he said on ITV. “But equally, it’s quite difficult to see how Labour could put together a coalition. It’s on a real knife edge.”

He said there would be a “huge postmortem” about a manifesto drawn up by a very small circle of people in Downing Street and the overall style of the election campaign.

Osborne suggested that Boris Johnson, the foreign secretary, would have “a little smile on his face right now” given the boost to his chances of taking over as Tory leader.

Sir Craig Oliver, David Cameron’s former director of communications at Downing Street, came to a similar verdict on Sky News.

“If this is true, if this is accurate, in CCHQ there will be deep and lasting shock,” he said. “It was the biggest gamble a politician has taken for a long time and if that exit poll is right, it’s failed.”

On the basis of the first few results, the seats of some Conservative big hitters were under threat, including the home secretary, Amber Rudd, in Hastings and Rye, and Anna Soubry, a prominent pro-EU former minister, in Broxtowe. Jane Ellison, the financial secretary to the Treasury, was the first Tory minister to lose her seat, in Battersea, a strongly remain voting area, while Ben Gummer, the architect of the manifesto, lost in Ipswich.

Labour went on to take back surprise marginals in the south of England, Bedford, Bristol North West and Canterbury, which has been Tory for the last century. Meanwhile, the Conservatives had gained only one seat in England so far, Walsall South, in the Midlands.

Exit poll points to hung parliament in 2017 general election

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In an ominous sign for May, Liam Fox, the international trade secretary, would not guarantee that the prime minister would not have to resign. “It’s very early in the evening and we’ll have to wait and see,” he said.

The only pocket of hope for the Conservatives was Scotland, where the wing of the party led by Ruth Davidson ousted Angus Robertson, the SNP Westminster leader, in Moray and took another seat in Angus. Some Tories attributed this to Davidson’s more liberal outlook and cheery demeanour in contrast to May’s social conservatism and a campaign in which she was accused of being a “gloombucket” by the Daily Mail’s sketchwriter.

On Thursday night, senior Conservatives were already discussing potential replacements, with reports that Johnson was tapping up MPs about his prospects. Betting companies immediately began running odds on potential candidates to replace the Conservative leader, with Johnson in the lead, followed by Philip Hammond, May’s chancellor.

The foreign secretary, who was sidelined for much of the Tory campaign until the end, repeatedly dodged questions about whether he would like to lead the party and called on people to “contain themselves” until they had seen the final result. At his victory speech in Uxbridge and South Ruislip, he hinted that the Conservatives had been out of touch, saying they had to “listen to our constituents and listen to their concerns”.

One Conservative MP who held his safe seat said May would be “toast” if she had not managed to at least match Cameron’s result, with the possibility of a new leader installed if they were to attempt to run a minority government.

The defence secretary, Michael Fallon, was sent to Conservative headquarters to calm his party’s nerves, saying on television: “This is a projection, it’s not a result. These exit polls have been wrong in the past.” David Gauke, a loyalist Tory minister, insisted May’s job was not in question. “As things stand at the moment I think she is the right person for the job,” he said.

However, there were very few Tories on the airwaves and silence from party spin doctors as a number of strong holds and a few gains for Labour started coming through.

If May manages to get a majority, her leadership would still be weakened, as the Conservatives had been hoping for a lead in seats of up to 100.

Labour and Tories – finally united in shock over exit poll | John Crace

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The prime minister toured many Labour-held target constituencies with the aim of winning over the opposition’s pro-Brexit heartlands. But her proposed shake-up of social care, which meant people would have to pay for care in their homes out of the value of their property, went down extremely badly with voters, along with a proposal to abandon the triple lock on pensions and withdraw winter fuel payments from the wealthy.

In the campaign postmortem, Tory MPs will demand to know how a manifesto was published with a raft of unpopular policies and lack of sweeteners to appeal to the electorate.

Above all, the decision to put May front and centre will be challenged, given her wooden media performances and refusal to debate head to head with the Labour leader, Jeremy Corbyn.

Sensing her weakness, Labour and the Liberal Democrats are expected to go after the prime minister hard. A Labour source said: “If this exit poll is correct, Theresa May’s credibility is completely shot. As May said herself, if she lost just six seats in this election she would not be prime minister.”

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It is time for Theresa May to go, says Jeremy Corbyn – video

Emily Thornberry, the shadow foreign secretary, said May “should go, because I think she has manifestly failed”, while deputy leader Tom Watson said May was a “damaged politician whose reputation may never recover”.

Ukip was unhappy with May for a different reason, saying the prime minister had “put Brexit in jeopardy”.

The situation is complicated by the fact that Brexit negotiations are due to start in 11 days. Ken Clarke, the former Tory chancellor and pro-EU MP, said a hung parliament would be very difficult for the progress of the talks.

“The worst outcome for the United Kingdom would be a weak government and a hung parliament of any party and we just have to see where we get,” he said.

“If we continue with another parliament with a small majority then firstly we will have to have some deeper debates ... particularly on Brexit, and, actually, as we face some appalling difficulties in my opinion, this is a critical stage for us. Politics is changing.”